Human preferences for a friendly face may have guided the evolution of dog eye color, according to researchers. Ever since dogs were domesticated, roughly 15,000 to 50,000 years ago, humans have been selecting—whether consciously or not—for certain traits in their dogs.

Among them, it seems that humans have a preference for puppy eyes. Scientists have found that dogs have developed specific facial muscles that allow them to take on a sad expression, evoking a caring response in humans and thus giving dogs that particular advantage.

Now research shows that humans may have influenced dogs’ eye color, too. A study by scientists in Japan found that dark eyes are more common in domesticated dogs than in their wild “relatives” and that people perceive dogs with dark eyes as friendlier.

The team argues that dark eyes may have been preferred by humans, possibly unconsciously, during domestication from wolves to dogs.

However, Akitsugu Konno, a researcher from Teikyo University of Science, noted that later development in some breeds could also have affected the color of dogs’ eyes: for example, the blue coat color seen in breeds such as shelties and border collies are associated with blue eyes.

Writing in the journal Royal Society Open Science, the team describes how they compared iris color in 22 images of gray wolves of various coat colors and from different locations, as well as images of 81 domestic dogs, finding that the latter tended to be darker and red eyes.

The team found that dark-eyed dogs were rated higher in friendliness than light-eyed dogs and were considered more relaxed, sociable and dependent, and less aggressive, intelligent and mature.